The present invention relates to a marine structure intended to be supported by the sea bed. The marine structure may for instance be a platform suitable for use in connection with offshore hydrocarbon production. More particularly, but not exclusively, the marine structure is intended to be installed on offshore sites subjected to seismic activities. In particular, the present invention relates to foundation means supporting the marine structure on the sea bed.
A foundation system has previously been disclosed in U.K. Patent Specification No. 1.417.471. This specification describes a marine structure consisting of a base which is intended to be supported by the sea bed and has three or more cells, at least some of which have a bottom. The marine structure consists further of a superstructure projecting upwards away from the sea bed and several hollow downwardly open skirts which project downwardly from the base to a lower level than said cell bottoms. The skirts are formed as an integral unit with the base and have a length which enables them to be pressed down into the sea bed to cooperate directly with the skirts to support the marine structure in position on the sea bed. The skirts are located with at least some parts thereof extending along the periphery of the base, said parts being joined to form a continuous barrier wall along the periphery of the base. At least three of the skirts have a pumping system associated therewith such as to permit righting of the structure into an upright position.
The present invention relates in particular to a support for a marine structure to be submerged onto the submarine bed and anchored or founded in such a way that the structure will be able to withstand large forces in lateral direction without capsizing, tilting or undergoing structural damage.
Since the marine structure particularly is designed to operate in areas subjected to seismic activities, the support will be subjected to two major types of environmental forces. These are:
(a) forces due to wind and wave action, and
(b) forces imposed on the foundation due to earthquakes.
A marine structure according to the present invention may for instance be designed to operate at depths exceeding 100 ft. The horizontal forces imposed on the structure due to wave action at such depths may be of the order of 30,000 to 60,000 tonnes while the moments very well may exceed 2,000,000 tonnes-m at the mud-line. These forces are to be transferred through the support or foundation to the sea bed. Hence, the foundation means should be of a type which is rigid enough to support the marine structure in position, restraining the tendency of the platform to move in lateral direction or from tilting.
On the other hand, a support or foundation means which is flexible enough to reduce the forces imposed by seismic activities on the foundation means (and the structure), is required.